1. Field of the Invention
The invention relates to a particulate matter detecting sensor and a particulate matter detecting apparatus. More particularly, the invention relates to a particulate matter detecting sensor that is arranged in an exhaust passage of an internal combustion engine and detects an amount of particulate matter in exhaust gas, as well as to a particulate matter detecting apparatus that uses this particulate matter detecting sensor.
2. Description of the Related Art
Published Japanese Translation of PCT application No. 2006-515066 (JP-A-2006-515066), for example, describes a sensor that detects an amount of particulate matter (PM) in exhaust gas of an internal combustion engine. This sensor has electrodes arranged parallel to one another with a space therebetween. This sensor is arranged in an exhaust passage with at least a portion of the electrodes exposed to the exhaust gas. When exhaust gas passes through the exhaust passage, particulate matter in the exhaust gas accumulates on the electrodes. As a result, the impedance between the electrodes changes. The sensor described above detects this change in impedance, and detects the amount of PM accumulated between the electrodes according to this change.
Incidentally, particulate matter accumulates on the electrodes of the sensor while the PM amount is being detected. If the amount of accumulated particulate matter increases and exceeds a certain accumulation amount, the sensor will not be able to output an output value equal to or greater than that threshold value. Therefore, in order to accurately detect the PM amount, the particulate matter that has accumulated on the electrodes at a certain stage must be removed. One known way to remove particulate matter that has accumulated on a sensor is to burn off the particulate matter by heating the sensor with a heater that is built into the sensor, as described in Japanese Patent Application Publication No. 2009-144512 (JP-A-2009-144512).
When removing particulate matter that has accumulated on a sensor by burning it off, the burn-off rate is limited by the oxygen concentration in the exhaust gas. Therefore, it may take a long time to burn off the particulate matter. However, while particulate matter is being removed, the sensor is unable to be used to detect the PM amount as it is normally, so having the burn-off process for removing the particulate matter take a long time is undesirable.
Also, with the removal method that burns off the particulate matter, carbon (C) that has bonded to the electrodes of the sensor may not be sufficiently removed. Therefore, if the sensor continues to be used after burning off, i.e., removing, the particulate matter, the carbon remaining on the electrodes will gradually increase, which may cause the zero point of the sensor output to become off. If this happens, the PM amount obtained according to the sensor output will be off. Therefore, a removal method that also enables carbon that has bonded to the electrode to be sufficiently removed is desirable.